Recently, there has been an increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, extensive research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion due to the use of an organic electrolyte solution, and are disadvantageously complicated to fabricate.
Many companies have produced a variety of lithium secondary batteries with different safety characteristics. It is very important to evaluate and ensure the safety of such lithium secondary batteries. In order to ensure such a safety, there has been proposed a separator having a porous coating layer formed by coating a mixture of inorganic particles and a polymer binder on at least one surface of a porous substrate having multiple pores. Also, in a recent separator preparation technique, separators have been prepared by applying the composition or thickness of the outermost porous coating layers asymmetrically, so that the surfaces of the separators to come into contact with a cathode and an anode are suitably matched with each of the two electrodes.
Meanwhile, the secondary batteries are classified according to the structure of an electrode assembly consisting of cathode/separator/anode, for example, into a jelly-roll (winding type) structure obtained by interposing a separator between a cathode and an anode which are in the form of a long sheet, followed by winding, or a stack-folding type structure obtained by winding bi-cells or full-cells with a long separator sheet continuously, the bi-cells or full-cells being obtained by laminating cathode units and anode units between which separators are interposed.
When the stack-folded electrode assembly is prepared according to the known method, the above-mentioned separator applying the composition or thickness of porous coating layers asymmetrically is difficult to be used because the selective contact of the separator with a cathode and an anode to be matched cannot be completely made due to the structural characteristic of the electrode assembly.
Meanwhile, in the jelly-roll type electrode assembly, the above-mentioned separator applying the composition or thickness of porous coating layers asymmetrically may be used to come into contact with a cathode and an anode selectively when considering the structure of this electrode assembly. However, since the jelly-roll type electrode assembly is prepared in by winding a cathode and an anode being a sheet form in the contact state thereof to form a cylindrical or oval cross-section, the electrode assembly is internally accumulated with stress caused by the expansion and contraction of electrodes during charging and discharging processes. When such a stress accumulation exceeds a certain limit, the electrode assembly is apt to be deformed. By the deformation of the electrode assembly, the space between the electrodes become un-uniform to deteriorate battery performances rapidly, and an internal short circuit is generated to threaten battery safety. Also, the winding of the cathode and the anode being a sheet form is difficult to maintain the uniform distance between the cathode and the anode, and the rapid winding thereof is also difficult, and thus it is unfavorable in terms of productivity.